The ability to accurately measure in vivo tendon forces would have a broad impact on studying tissue properties, advancing assistive technologies, and furthering our scientific understanding of the human neuromuscular control system. Myoelectric prosthetics and functional electrical stimulation devices could utilize closed-loop control strategies, resulting in restored motor function in disabled populations. Furthermore, researchers could accurately study soft tissue properties, neuromuscular function, and motor performance directly, rather than having to rely on inaccurate, numerical approximations of these systems. It may also be possible for miniature in vivo sensors to provide feedback during surgical procedures such as limb re-attachment and cardiac treatments. During Phase I of this project, a novel optically based sensor was developed that has shown great promise in achieving this goal. Our device, which can be miniaturized to less than 500 microns in diameter, is based on a modified fiber Bragg grating (MFBG) optical strain sensor. Through a series of experiments that included testing synthetic, animal, and human tendons, the MFBG sensor has demonstrated the ability to accurately measure tendon forces with a number of important advantages over other techniques. These include, (1) the ability to measure tendon forces without being influenced by skin artifacts that have plagued optically based approaches in the past, (2) the ability to measure very localized forces, (3) automatic compensation of temperature variations and (4) the ability to control the size and sensitivity of the sensor depending on the application. We are confident that this new sensor will result in fundamental and scientific advances in both research and commercial settings. The general aim of this Phase II project is to construct and test a robust commercially viable measurement system based on the new optical force sensor developed during Phase I. At the conclusion of the Phase II project our intention is to have a system proven to be safe, effective and ready for human testing. [unreadable] [unreadable] [unreadable]